I'm intrigued, while knowing that there is no way of sorting out my intrigue any further ...in Breton folklore there is a character known as a Marie-Morgan, sometimes found by the sea and sometimes in a forest by a well or a spring, who sings to attract the attention of a passing young man. When she has attracted him, she embraces him and pulls him with her into the water, thus inadvertently drowning him, which means she has to go back above ground to sing again. In Arthurian romances there is the Lady of the Fountain, who has a white marble slab on which water can be poured, and when it is it triggers a violent storm follwed by the arrival of a knight in armour who must be fought. In the Occitan romance I'm working on for my PhD there is another fairy woman, almost certainly Morgan le Fay (but not named as such) who needs the help of the hero of my romance and to get it she calls for help near a fountain and then pushes the hero into the water as her kingdom is under water. Luckily he doesn't drown (although everyone above ground thinks he does, until he returns).
So although none of this is much like any of your ballads I have (a) a white marble slab, (b) a fairy who sits by a fountain and who might lure a mortal man to his doom (or not) and (c) a fairy who lives in the water. As well as a potential slayer for le roi Renaud, although my memory of the lyrics to that is that he's coming back from battle rather than an aquatic home-breaker.

I'm also just starting to read an academic book which is looking at the links between ballads and medieval texts....

Ignore me. It's too hot and I'm probably raving.